If you are receiving this error then please ensure that your files directory has write permissions for the httpd service. The most secure setting would be to ensure the group for the files directory is the same as the httpd service owner and give the group on the files directory read, write and execute permissions. If you have the correct permissions and still receive this error then please tell us in a comment below.
I have set up a Drupal 7.12 on a local LAMP running on Debian for a restailying of a site of mine.
Trying to use XML sitemap 7.x-2.0-rc1 and I get the following error while trying to generate the sitemap:
Warning: XMLWriter::openUri(): Empty string as source in XMLSitemapWriter->openUri() (line 40 of /srv/www/silicongadget.com/public_html/modules/xmlsitemap/xmlsitemap.xmlsitemap.inc).
Please tell me if you need any particular info.